[range.zip.transform.iterator] Avoid "defining" iterator_category

Use "declare" instead.

Author: frederick-vs-ja

source/ranges.tex

@@ -11887,7 +11887,7 @@
 \pnum
 The member \grammarterm{typedef-name}
 \tcode{\exposid{iterator}::iterator_category}
-is defined if and only if \exposid{Base} models \libconcept{forward_range}.
+is declared if and only if \exposid{Base} models \libconcept{forward_range}.
 In that case,
 \tcode{\exposid{iterator}::iterator_category} is defined as follows:
 \begin{itemize}